1. Field of the Invention
The present invention is directed to a method of producing weld metal tubing employing a reusable mandrel, and the tubing produced by said method.
2. Background of the Invention
Heavy or thick-walled tubing or pipe finds various uses in several industries. One approach to manufacturing such thick-walled tubing is by a process involving depositing a weld metal onto an exterior surface of a thin-walled starter tube. The tubing produced by this weld metal buildup is frequently referred to as weld metal tubing.
Weld metal tubing is conventionally produced by forming a flat steel sheet or plate into a cylindrical shape which serves as a starter tube. The tube wall thickness of the starter tube is, therefore, substantially the thickness of the flat plate from which it was formed. The tube wall thickness is then increased by depositing or overlaying a weld metal onto the exterior surface, thereby "building up" the outside diameter (and wall thickness) of the tube.
The thick-walled tubing produced in this manner is not fully satisfactory for use in all applications because the weld metal buildup process leaves distortions in the final product. The heat generated in the weld metal buildup process and the subsequent cooling of the weld metal from its solidus temperature to a weld interpass temperature creates in the tube diametrical shrinkage, surface distortion, bowing along the length of the tube, and longitudinal shrinkage of the tube.
These distortions imparted by the process are undesirable and in some applications render the tubing unusable. The presence of distortion is thus a limiting factor in the types of applications for which the tubing could be used.
One use envisioned for a thick-walled tubing produced by weld metal buildup is as a low-cost alternative to, or replacement for, more expensively produced molds used for centrifugal casting of pipe. However, because the interior surface of the weld metal tubing would serve as the form for the exterior wall of the cast pipe, the surface is required to be substantially free from distortion.
Substantially distortion-free weld metal tubing may be achieved by a process wherein the weld metal is deposited in thickness greater than the desired finished thickness, and subsequently machining the interior and/or exterior surface of the tubing to restore the roundness and arrive at a finished product shape. This technique possesses major disadvantages in that additional weld metal material is required, and an additional, highly time consuming machining step is introduced. The cost and increased time to produce such tubing presents significant disadvantages.
Mandrels have been used heretofore to provide a surface against which materials can be formed. The mandrel is used to assist in shaping a product, in many instances to take on the contour of the mandrel. A standard mandrel of this type would not, however, provide a solution to the distortion problems in the manufacture of weld metal tubing. A cylindrically shaped mandrel could indeed provide a true surface over which to form a section of weld metal tubing, however the diametrical shrinkage experienced when the heating and subsequent cooling takes place in depositing the weld metal would create a substantial interference fit between the tubing and the mandrel. This would make removal of the mandrel very difficult, if not impossible, without employing destructive measures.
It is therefore a principal object of the present invention to provide a method for consistently and repeatedly producing weld metal tubing which is relatively free of distortion.
It is a further object of the present invention to provide a method of producing thick-walled tubing in a more cost-effective manner than previously employed.
It is further object of the present invention to provide a substantially distortion-free weld metal tubing product made by the method of the present invention which is substantially free from distortion.